The present invention relates to a clutch for controlling the transfer of power in a transmission, and more particularly to a device for controlling the operation of such a clutch.
Clutches for controlling the transfer of power in transmissions are classified into various types dependent on the transmissions with which the clutches are to be combined. For example, some clutches used in gear transmissions employ friction clutch plates, and those used in hydraulically operated transmissions comprise clutch valves for controlling the opening of a bypass passage.
One clutch device for use in a hydraulically operated continuously variable transmission is disclosed in Japanese Laid-Open Patent Publication No. 56-95722, for example. The disclosed clutch device includes a closed hydraulic circuit interconnecting a hydraulic pump and a hydraulic motor and composed of two hydraulic passages with a bypass passage extending therebetween, and a clutch valve disposed in the bypass for adjusting the opening of the bypass passage. The clutch valve is controlled in operation by a hydraulic actuator unit in the form of a cylinder. The hydraulic actuator unit is arranged to generate a control force commensurate with the rotational speed of an engine coupled to the transmission and acting in a direction to close the clutch valve (i.e., to connect the clutch), and a control force commensurate with the opening of the throttle valve and acting in a direction to open the clutch valve (i.e., to disconnect the clutch).
When the accelerator pedal of a motor vehicle with the transmission installed thereon is depressed to increase the throttle valve opening for setting the motor vehicle started, the rotational speed of the engine increases beyond a certain speed level, and the clutch valve is closed to connect the clutch. The motor vehicle is thus smoothly started.
If a hydraulic servo unit is employed to control the operation of the clutch valve to close the same, then it is necessary supply working oil of a certain hydraulic pressure to a cylinder chamber in the hydraulic servo unit for closing the clutch valve and also to drain working oil from an opposite cylinder chamber in the hydraulic servo unit.
If the working oil were discharged or drained without any restriction, then the clutch valve would be closed so quickly that the clutch would be connected abruptly, thereby producing a shock. It would be possible to provide an orifice in a drain passage for restricting the flow of the working oil discharge, so that the clutch valve would be closed at an appropriate speed by the hydraulic servo unit. With the orifice merely provided in the drain passage, however, since the rate at which the working oil flows through the orifice differs depending on the temperature of the working oil, the clutch would be connected slowly at a low oil temperature, and any desired clutch control would be impossible to carry out. Such a problem could be solved by varying the cross-sectional area of the orifice depending on the temperature of the working oil, but a complex mechanism would be needed to adjust the cross-sectional area of the orifice.
The same drawbacks would also be experienced with the controlling of a clutch having friction clutch plates with a hydraulic servo unit.
Adequate values required for the speed at which the clutch valve is closed vary from throttle valve opening to throttle valve opening. It is difficult to maintain the valve closing speed at an adequate level simply by providing the orifice in the drain passage.
Generally, when the throttle valve opening is small, i.e., if the clutch is to be connected while the accelerator pedal is not depressed substantially, it is required that the clutch valve be closed quickly to connect the clutch quickly. Conversely, when the throttle valve opening is large, the clutch has to be connected relatively slowly.
The above clutch valve control will be described in greater detail with reference to the continuously variable transmission disclosed in the publication referred to above. In the disclosed continuously variable transmission, after the clutch has been completely connected, a target engine rotational speed is established which corresponds to the throttle valve opening, and the transmission ratio or speed reduction ratio of the transmission is controlled so that the actual engine rotational speed will be brought into accord with the target speed. In order to effect a smooth transition from the connection of the clutch to the control of the speed reduction ratio, it is desirable so that the engine speed will not exceed the target speed when the clutch is connected, and the engine speed will smoothly be increased during the transition from the connection of the clutch to the control of the speed reduction ratio. If the clutch is connected when the accelerator pedal is not deeply depressed and hence the throttle valve opening is small, since the target engine rotational speed is low, the actual engine speed becomes higher than the target speed and then is lowered to the target speed after the clutch is connected, unless the clutch is connected quickly.
The clutch may be connected quickly by increasing the cross-sectional area of the orifice and closing the clutch valve quickly. By doing so, however, the clutch is connected too quickly when the accelerator pedal is depressed to a large degree and the throttle valve opening is large, and the engine speed is subjected to hunting.